[llvm] [mlir] [NFC][TableGen] Migrate IfDef/Namespace emitter from MLIR to LLVM (PR #161744)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> There likely should be unit-tests for this in LLVM

Do you mean lit tests or C++ unit tests? I don't see a lot of C++ unit tests for TableGen (just 3 in unittests/TableGen) but I can add one for these 2 classes. Note that td lit tests also exercise this.

https://github.com/llvm/llvm-project/pull/161744